In normal operation, the speed of the turbogenerator in such a system is controlled by exporting power to the local electricity grid. If the turbogenerator speed begins to increase then the output power is increased to slow the turbogenerator down. By this means, the turbogenerator is held at a constant speed. However, in the event of a grid disturbance or outage, the turbogenerator would accelerate uncontrollably, in the absence of any additional controls.
As is well known, the electrical grid is affected by disturbances, which may last from a few micro-seconds to periods of downtime or outage lasting for seconds, minutes or even hours. Most countries have national or state regulations, which require generators to disconnect from the electrical grid if the grid voltage or frequency falls outside certain limits for more than a predetermined period, typically a few hundred milli-seconds, depending on the utility that is receiving the output power. For shorter disturbances or outages, up to the time limit allowed by the utility, it is important for the generator to continue working during the disturbance.